The magnesium crystal has a structure of close-packed hexagonal, and the Magnesium crystal sheet with strong textures exhibits mechanical properties of anisotropy and low formability. A fine grain structure and a disperse weak texture are the basic solution of improving the deformability under the conditions of medium-low temperatures and rapid strain rates, and of reducing the anisotropy of deformation, and at the same time, this micro-structure can improve the surface quality of the formed magnesium sheet. During the plastic deformation of magnesium alloy, the fine grain structure can restrict the occurrence of mechanical twin crystals effectively, alleviate moderately the demands of multi-crystal continuous deformation on the dislocation gliding coefficient number through grain boundary sliding, with reducing the over-stress concentration at the local grain boundary and accommodating the deforming defects; disperse weak sheet textures can increase the base surfaces and the cylinder surfaces to activate the sliding motion, improve the deformation hardening index and enable the deformation to occur evenly along the sheet surface, so as to enhance the formability.
The fine grains and disperse weak textures can be obtained by appropriate rolling technologies. Hitachi Metals carries out rolling at high temperature (about 500° C.), which starts the slipping of non-basal surfaces (Prismatic <a> and Pyramidal <c+a>) at the same time. The strength of the textures of magnesium sheet is 3.7, and the grains before and after annealing is kept substantially at about 6 μm, such that the sheet can be stamped at the room temperature.
US NanoMag Company produces AZ61 magnesium sheets by the way of rolling above the dynamic recrystallization, preheating the rollers up to 200° C., adopting the deforming mode of a large reduction rate in a single-pass (>40%), with the strength of the basal surface texture of the material being less than 3. The sheet texture after annealing is further weakened and diffused, with the micro-structure being isometric crystal; it should be noted that the particles of intermediate phase diffused by the AZ61 magnesium alloy matrix promote the weakening of the texture of the rolled sheet.
Japan Osaka University proposes the deforming mode of “high strain rate, large reduction rate per pass”, with the strain rate of 180-2000/s and the reduction rate per pass of 50-60%. In the rolling deformation area, the heat from rolling deformation drives the rolling temperature to rise up obviously, so as to generate dynamic recrystallization. The materials consist primarily of isometric crystals of a dimension of 5 μm and the sheet texture becomes diffused.
The technical route of the magnesium alloy rolling process for obtaining the fine grains and the disperse weak textures, are briefly summarized as follows: 1) rolling at high temperature; 2) high strain rate, large reduction rate per pass; 3) shear rolling, 4) repeatedly bending and leveling after rolling.
An alloy design is another way to obtain magnesium sheets with fine grains and disperse weak textures, Korean patent KR2003044997 discloses a high formability magnesium alloy and a method of producing the same, which has the chemical compositions (in weight percentage): Zn: 0.5˜5.0%, Y: 0.2-2.0%, Al: less than or equal to 2.5%, Mn: less than or equal to 0.5%, Ti: less than or equal to 0.2%, Zr: less than or equal to 0.5%, Cd: less than or equal to 0.5%, Tl: less than or equal to 0.5%, Bi: less than or equal to 0.5%, Pb: less than or equal to 0.5%, Ca: less than or equal to 0.3%, Sr: less than or equal to 0.3%, Sn: less than or equal to 0.5%, Li: less than or equal to 0.5%, Si: less than or equal to 0.5%; the technical processes thereof are: 1) heating the magnesium ingot up to 250˜450° C. for a heating time of 2 min/mm; 2) rolling at a temperature of 200˜450° C., with the first-pass reduction rate being less than or equal to 20%, and the other-pass reduction rate being 10˜35%; 3) annealing at the temperature of 180˜350° C.
China patent CN101985714 discloses a high-plasticity magnesium alloy and a method of preparing the same, which has the chemical compositions (in weight percentage): Al: 0.1˜6.0%, Sn: 0.1-3.0%, Mn: 0.01-2.0%, Sr: 0.01-2.0%, and which can be used for producing sheets and sections.
Japan patent JP2012122102A discloses a high-formability magnesium alloy which has the chemical compositions (in weight percentage): Zn: 2.61-6.0%, Ca: 0.01-0.9%, and a trace of Sr and Zr, wherein preferably the total contents of Ca and Sr is between 0.01˜1.5%, and the total contents of Zr and Mn is between 0.01-0.7%; the produced magnesium sheet has the room temperature properties: the yield strength of 90 Mpa, and the Ericksen value of more than or equal to 7.0.
WO2010110505 discloses a method of manufacturing Mg—Zn-based magnesium alloy with high-speed formability at room temperature, which has the chemical compositions (in weight percentage): Zn: less than or equal to 3.5%, and one or more elements of Fe, Sc, Ca, Ag, Ti, Zr, Mn, Si, Ni, Sr, Ni, Sr, Cu, Al, Sn; and which material presents excellent formability through lowering the recovery and recrystallization temperatures and activating the slippage of the low-temperature non-basal surfaces.
Recently, Korean patent KR20120049686 discloses a high-strength high-formability magnesium sheet and a method of producing the same, which has the chemical compositions (in weight percentage): Zn: 5-10%, Ag: 0.1-3.0%, Ca: 0.1-3.0%, Zr: 0.1-3.0%, Mn: 0.1-1.0%; wherein fine structures can be obtained via the pretreatment before rolling and TMP technology, and the limit forming height may be beyond 10 mm.
Rare earth elements can weaken the texture of the magnesium alloy sheet. For instance, in the patent WO2010041791, Y elements are added into the Mg—Zn-based magnesium alloy to generate the effects of precipitation strengthening and the twin-roller continuous casting and rolling and TMP technology are employed for refine grains. The obtained material has the advantages of high strength, plasticity, and low anisotropy at the room temperature, thereby presenting high formability.
Additionally, the textures of the rare earth magnesium alloy sheet such as ZE10 (Mg1.3Zn0.1Ce), ZEK100 (Mg1.3Zn0.2Ce0.1La0.5Zr), ZW41 (Mg4.0Zn0.7Y), ZG11 (Mg1.2Zn0.8Gd), ZG21 (Mg2.3Zn0.7Gd), are weakened obviously. Taking ZG11 as an example, it has a grain size of 12-15 μm, an uniform elongation rate of 15%, a total elongation rate of up to 36% and Lankford value of 1 (far lower than AZ31:3), with reference to H Yan etc., Mater. Sci. Eng. A, 2010, 527: 3317-22.
Although the rare earth elements work well in weakening the texture of magnesium sheet, but taking factors such as the cost into account, it is difficult for rare earth magnesium alloy sheets to be applied into automobiles. For the fields of automobiles and rail transits, it is required that the alloy design and production processes be simple and effective, and the performances be “proper” rather than “excellent”, with a balance among the lightweight, performances and cost, which is totally different from the field of military, aerospace, etc.